Method of manufacturing silicon wafer

ABSTRACT

A method of manufacturing a silicon wafer including the step of flattening fine roughness existing on a side face of a silicon block or a silicon stack used for manufacturing the silicon wafer.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is related to Japanese application Nos.2000-296628 and 2001-272356, filed on Sep. 28, 2000 and Sep. 7, 2001,whose priority is claimed under 35 USC § 119, the disclosure of which isincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a method of manufacturing asilicon wafer. In particular, it relates to a polishing technique forflattening fine roughness existing on a side face of a silicon block ora silicon stack.

[0004] 2. Description of Related Art

[0005] Demand for silicon wafers is increasing year by year inaccordance with the spread of solar cells and the like. For example, onesolar cell requires about 54 silicon wafers of 5×5 inch square, whichare much greater than the number of silicon wafers required in IC andLSI.

[0006] The silicon wafer includes polycrystalline and single crystallinesilicon wafers, which are manufactured by the following method.

[0007] The polycrystalline silicon (polysilicon) wafer is obtained bymanufacturing a square polysilicon ingot, cutting the ingot into pluralpolysilicon blocks 1 with a band saw 20 (FIG. 4) and slicing thepolysilicon block 1 (FIG. 5). FIGS. 4 and 5 show a side face 19 of asilicon block, an edge 21 of a silicon block and silicon wafers 46.

[0008] The single crystalline silicon wafer is obtained by cutting acylindrical silicon ingot manufactured by a crystal pulling method(generally 1 m in length) into cylindrical single crystalline siliconblocks in a suitable size (generally 40 to 50 cm in length), grindingthe single crystalline silicon block to form a flat portion called anorientation flat and slicing the silicon block.

[0009] Where the silicon wafer of high dimensional accuracy is required,grinding is carried out in both cases of processing the polycrystallineand single crystalline silicon blocks. Specifically, the grinding isperformed by rotating a polishing wheel 45 such as a circular grindstonecontaining abrasive grains or a diamond wheel at high speed, pressingthe silicon block 1 onto the polishing wheel and moving them relativelyto each other. FIG. 6 shows a one-axis stage 7, a direction 11 alongwhich the stage 7 moves, a motor 5 for rotating the polishing wheel, atwo-axes stage 6 and a direction 10 along which the stage 6 moveslaterally.

[0010] In a conventional process of manufacturing the silicon wafer, aprocess of improving dimensional accuracy of the silicon block or thesilicon stack, or a process of erasing unevenness on the surface of thesilicon block or the silicon stack has been carried out. However,flattening of the fine surface roughness existing on its side faces hasnot been conducted.

[0011] The thus obtained silicon wafer is subjected to processing of aside face (may be referred to as a periphery face or a circumferentialface).

[0012] The periphery processing is carried out by grinding the peripherysurfaces of the silicon wafers one by one into a desired configurationin the same manner as a method of processing a glass substrate describedin Japanese Unexamined Patent Publication No. Hei 10 (1998)-154321, orby chemical polish (etching).

[0013] Since the solar cell requires a large number of silicon wafers ascompared with IC and LSI, the above-described periphery processing withrespect to each of the silicon wafers consumes a lot of time, investmentin equipment and labor. This may delay the supply of the silicon wafersbehind the demand. Further, the etching requires equipment for liquidwaste treatment, which also involves a problem of equipment costs.

[0014] However, without the periphery processing, the silicon wafer maybe cracked in a later step for manufacturing the solar cell, whichreduces a product yield. Accordingly, there has been demandeddevelopment of an efficient method for the periphery processing.

SUMMARY OF THE INVENTION

[0015] Accordingly, the present invention provides a method ofmanufacturing a silicon wafer comprising the step of flattening fineroughness existing on a side face of a silicon block or a silicon stackused for manufacturing the silicon wafer.

[0016] According to the present invention, the side face of the siliconblock or the silicon stack is flattened to such an extent thatdimensional accuracy is improved and surface unevenness is eliminated,i.e., the side face is flattened so that it has surface roughness Ry of8 μm or less, preferably 6 μm or less.

[0017] These and other objects of the present application will becomemore readily apparent from the detailed description given hereinafter.However, it should be understood that the detailed description andspecific examples, while indicating preferred embodiments of theinvention, are given by way of illustration only, since various changesand modifications within the spirit and scope of the invention willbecome apparent to those skilled in the art from this detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a schematic view illustrating a method of manufacturinga silicon wafer according to Method 1 of the present invention;

[0019]FIG. 2 is a schematic view illustrating a method of manufacturinga silicon wafer according to Method 2 of the present invention;

[0020]FIG. 3 is a graph illustrating a relationship between surfaceroughness of a circumferential surface of a silicon wafer and crackingreduction ratio of a solar cell from the silicon wafer;

[0021]FIG. 4 is a schematic view illustrating a method of cutting asilicon ingot into silicon blocks;

[0022]FIG. 5 is a schematic view illustrating a method of slicing asilicon block into silicon wafers; and

[0023]FIG. 6 is a schematic view illustrating a conventional process ofgrinding a silicon block.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] An object of the present invention is to provide a polishingtechnique for flattening fine roughness existing on a side face of asilicon block or a silicon stack in a short period so that the siliconwafer is prevented from cracking and improved in yield.

[0025] As a result of eager researches for solving the above problems,it has been found that the fine roughness on the side face of thesilicon block or the silicon stack causes the cracking of the siliconwafer and decreases the yield. Then the inventors have found that thecracking is prevented and the yield is improved efficiently byflattening the fine roughness before slicing the silicon block or thesilicon stack into the silicon wafer. Thus, the present invention hasbeen achieved.

[0026] The “silicon stack” mentioned in the present applicationsignifies a silicon block in the shape of cylinder or quadratic prism inwhich two or more silicon wafers are stacked. The “side face of thesilicon block or the silicon stack” mentioned in the present applicationsignifies a face which will constitute a circumferential surface of thesilicon wafer.

Method 1

[0027] According to Method 1 of the present invention, a mixture ofabrasive grains and a medium is sprayed on a side face of the siliconblock or the silicon stack, a polishing member is shifted closer to orcontacted with the side face to be polished, and the silicon block orthe silicon stack is moved relatively to the polishing member in thepresence of the abrasive grains so that the side face of the siliconblock or the silicon stack is mechanically and physically polished.Thereby the fine roughness existing on the side face of the siliconblock or the silicon stack is flattened.

[0028] The abrasive grains may be known abrasive grains, e.g., diamond,GC (Green Carborundum), C (Carborundum), CBN (cubic boron nitride) andthe like.

[0029] The medium to spray the abrasive grains may be a liquid such aswater, alkaline solution, mineral oil, glycols (polyethylene glycol,propylene glycol (PG)) or the like, or a gas such as air or inert gas,e.g., nitrogen, helium, neon, argon or the like. The abrasive grains maybe mixed in a ratio of about 0.5-1.5 kg with respect to 1 kg of theliquid medium or about 0.01-2:1 kg with respect to 1 liter of thegaseous medium.

[0030] The polishing member may be made of steel, resin, cloth, spongeor the like. More specifically, it may be a steel brush, a resin brush,a sponge wheel or the like. The polishing member may or may not have theabrasive grains on its surface and/or in the inside thereof.

[0031] Method 1 will be detailed with reference to FIG. 1.

[0032] A polishing member 13 is arranged on a polishing wheel 4 so thatit contacts with a side face 9 of a silicon block 1 to be polished, andthen rotated at high speed by a motor 5 for rotating the polishing wheelalong a direction 12 shown in FIG. 1. At this time, a mixture 8 ofabrasive grains 14 and a medium 15 (may be referred to as “slurry” or“dispersed abrasive grains”) is sprayed from a nozzle 3. Further, thesilicon block 1 is reciprocated by a one-axis stage 7 along a direction11 shown in FIG. 1. According to the rotational movement of thepolishing wheel 4 and the reciprocal movement of the one-axis stage 7,the side face 9 is entirely polished and the fine roughness is removed.The slurry 8 is used to let the abrasive grains 14 into the polishingmember 13 of the polishing wheel 4 so that the side face 9 is polishedwith the abrasive grains 14. Further, the medium 15 in the slurry 8serves to discharge silicon shavings and unnecessary abrasive grains 14,and cool the side face 9.

[0033]FIG. 1 shows a two-axis stage 6 capable of moving in a lateraldirection 10 and a vertical direction 31, which is used to shift thepolishing wheel 4.

Method 2

[0034] According to Method 2 of the present invention, a medium issprayed on a side face of the silicon block or the silicon stack, apolishing member having abrasive grains on its surface and/or in theinside thereof is shifted closer to or contacted with the side face tobe polished, and the silicon block or the silicon stack are movedrelatively to the polishing member so that the side face of the siliconblock or the silicon stack is mechanically and physically polished.Thereby the fine roughness existing on the side face of the siliconblock or the silicon stack is flattened.

[0035] The medium to spray the abrasive grains may be theabove-described liquid or gas. The liquid or the gas may not contain theabrasive grains.

[0036] The polishing member having the abrasive grains on its surfaceand/or in the inside thereof may be made of steel, resin, cloth, spongeor the like having, on its surface and/or in the inside thereof,abrasive grains such as diamond, GC (Green Carborundum), C(Carborundum), (CBN (cubic boron nitride) or the like. Moreparticularly, the polishing member may be a steel brush, a resin brush,a sponge wheel or the like.

[0037] The liquid or the gas to be sprayed serves to remove, from thesurface of the silicon block, silicon shavings and the abrasive grainsfallen from the surface and/or the inside of the polishing member. Wherethe liquid or the gas containing no abrasive grains is used, the liquidor the gas is easily recycled and the abrasive grains and the siliconshavings are easily separated.

[0038] Method 2 will be detailed with reference to FIG. 2.

[0039] The difference from Method 1 is that the polishing member 17having the abrasive grains on its surface or in the inside thereof isarranged on the polishing wheel 4 so that it contacts with the side face9 of the silicon block 1 to be polished, and then a polishing liquid orpolishing gas 16 comprising a medium 18 is sprayed. That is, the sideface 9 of the silicon block 1 is polished by the abrasive grains 14 (notshown) of the polishing member 17. The polishing liquid or polishing gas16 is sprayed onto the side face 9 of the silicon block 1 to be polishedin order to remove the silicon shavings, unnecessary abrasive grains(grain scraps) and waste generated during the polishing, and to cool theside face 9. Other components than the above-mentioned ones areindicated by the same reference numbers shown in FIG. 1.

[0040] This method prevents contamination of the side face by thesilicon shavings, grain scraps and waste, and sticking of suchunnecessary wastes to the side face after polishing. Accordingly,reduction of processing quality is prevented. Where the polishing liquidis used, the removal of the shavings and waste can be easily carried outby using a filter or the like, which eliminates the need to exchange theliquid in every polishing process.

[0041] The side face of the silicon block or the silicon stack flattenedby the above method preferably shows surface roughness Ry of 8 μm orless, more preferably 6 μm or less. Where the thus obtained siliconblock or the silicon stack having the surface roughness of 8 μm issliced into silicon wafers for manufacturing a solar cell, the yield ofthe solar cells increases because damage of the silicon wafers is small.

[0042] In the method of manufacturing the silicon wafer according to thepresent invention, the section of the silicon block or the siliconstack, i.e., the shape of the silicon wafer in a front view, is notparticularly limited. However, it is preferred that the sectioncomprises four main lines and the lines form angle of about 90° withadjacent lines, respectively. That is, the section is preferably arectangle or almost rectangle constituted of sides parallel to oppositesides, respectively. The silicon block or the silicon stack having sucha section is preferred because two opposite side faces can be polishedand flattened simultaneously. This allows high-speed processing.Further, where the silicon block or the silicon stack has a rectangularor almost rectangular section, accuracy in positioning the polishingwheel and the silicon block or the silicon stack is not required, whicheliminates the need of expensive equipment.

[0043] Alternatively, the rectangular or almost rectangular section ofthe silicon block or the silicon stack may be formed of four linesconnected to adjacent lines via another line or curve, respectively.That is, the section may have rounded corners each having a curve or anarc.

EXAMPLES

[0044] Hereinafter, the present invention will be further detailed byway of examples, but the invention is not limited thereto.

Example 1 (Cutting a Silicon Block)

[0045] As shown in FIG. 4, a silicon block 1 was cut from a siliconingot using a band saw 20. FIG. 4 shows a side face 19 of the siliconblock and an edge 21 of the silicon block.

[0046] Four side faces 19 of the silicon block 1 were flattened by themethod of the present invention to reduce defective wafers cracked in alater step and thus improve yield of the silicon wafer.

Example 2 (Method 1)

[0047] The silicon block 1 of 125×125×250 mm obtained in Example 1 waspolished by Method 1 to confirm the effect of the invention. A spongewheel and a mixture of GC abrasive grains (#800) with polish oil wereused as the polishing member 13 and the slurry 8, respectively.

[0048] As a result, four side faces 9 were polished in 16 minutes.Surface roughness Ry of the side faces was reduced from 20 μm to 5.8 μmby the polishing.

Example 3 (Method 1, Using Resin Brush)

[0049] The silicon block 1 of 125×125×250 mm obtained in Example 1 waspolished by Method 1 to confirm the effect of the invention. As thepolishing member 13, a wheel (240 mm in diameter) provided with nylonresin hairs (0.5 mm in diameter, 20 mm in length) densely fixed with anepoxy adhesive on a bottom region of 160-240 mm diameter was used. Asthe slurry 8, a mixture of GC abrasive grains (#800) and polish oil(weight ratio 1:1.28) was used.

[0050] The polishing member 13 was pressed on the surface of the siliconblock 1 to such a degree that the distal ends of the nylon resin hairsreach 1.5 mm below a position where the distal ends contact the surfaceof the silicon block 1. Then, the polishing member was rotated at 1800rpm.

[0051] After the polishing member 13 contacted the surface of thesilicon block 1, the silicon block 1 was moved along a lengthwisedirection of the silicon block, which is orthogonal to a rotation axisof the polishing member 13. The silicon block 1 was moved at 0.6 mm/sec.

[0052] From the circumference of the polishing member 13, the slurry 8of 150 l/min was sprayed onto the side face 9 of the silicon block 1 tobe polished.

[0053] As a result, four side faces 9 were polished in 12 minutes. Thesurface roughness Ry was reduced from 12 μm to 2.8 μm by the polishing.Cracking reduction ratio was 2.5 fold (ratio of cracked defective waferswas reduced by 60%, i.e., reduction of yield due to wafer cracking wasdecreased by 60%).

[0054] The cracking reduction ratio signifies a value obtained bydividing a ratio (X_(A)) of cracked silicon wafers to silicon wafershaving reference surface roughness Ry=A μm used for the manufacture of asolar cell panel by a ratio (X_(B)) of cracked silicon wafers to siliconwafers having surface roughness of Ry=B μm used for the manufacture of asolar cell panel (provided that A>B).

(Cracking Reduction Ratio)_(Ry=B)=(X _(A) /X _(B))

[0055] For example, provided that X₂₀=1 and X₈=0.66, the crackingreduction ratio is calculated as follows:

(Cracking reduction ratio)_(Ry=B)=(X ₂₀ /X ₈)=1/0.66=1.52

Example 4 (Method 2)

[0056] The silicon block 1 of 125×125×250 mm obtained in Example 1 waspolished by Method 2 to confirm the effect of the invention. A spongewheel provided with diamond grains (#800) was used as a polishing member17 and polish oil was used as a polishing liquid 16 containing noabrasive grains.

[0057] As a result, four side faces 9 of the silicon block were polishedin 14 minutes. The surface roughness Ry was reduced from 12 μm to 5.8 μmby the polishing.

Example 5 (Method 2, Using Resin Brush)

[0058] The silicon block 1 of 125×125×250 mm obtained in Example 1 waspolished by Method 2 to confirm the effect of the invention. As thepolishing member 17, a wheel (220 mm in diameter) provided with nylonresin hairs (0.4 mm in diameter, 15 mm in length) containing diamondgrains (#320) densely fixed with an epoxy adhesive on a bottom region of160-240 mm diameter was used. The slurry 8 used in Example 3 was used asthe polishing liquid 16.

[0059] The polishing member 17 was pressed on the surface of the siliconblock 1 to such a degree that the distal ends of the nylon resin hairsreach 1.5 mm below a position where the distal contact the surface ofthe silicon block. Then, the polishing member was rotated at 600 rpm.

[0060] After the polishing member 17 contacted the surface of thesilicon block 1, the silicon block 1 was moved along a lengthwisedirection of the silicon block 1 which is orthogonal to a rotation axisof the polishing member 17. The silicon block 1 was moved at 5 mm/sec.

[0061] From the circumference of the polishing member 17, the slurry 8of 150 l/min was sprayed onto the side face 9 of the silicon block 1 tobe polished.

[0062] As a result, four side faces of the silicon block were polishedin 4 minutes. The surface roughness Ry was reduced from 12 μm to 5 μm bythe polishing. The cracking reduction ratio was 2 fold (ratio of crackeddefective wafers was reduced by 50%, i.e., reduction of yield due towafer cracking was decreased by 50%).

Example 6 (Method 2, Using Resin Brush)

[0063] The silicon block 1 polished in Example 5 was further polishedfor 4 minutes to confirm the effect of the invention in the same manneras in Example 5, except that a wheel (220 mm in diameter) provided withnylon resin hairs (0.4 mm in diameter, 15 mm in length) containingdiamond grains (#800) and fixed densely with an epoxy adhesive on abottom region of 160-220 mm diameter was used as the polishing member17.

[0064] As a result, the surface roughness Ry was reduced from 12 μm to 1μm by the polishing. The cracking reduction ratio was 2.5 fold (ratio ofcracked defective wafers was reduced by 60%, i.e., reduction of yielddue to wafer cracking was decreased by 60%).

Example 7 (Surface Roughness and Cracking Reduction Ratio)

[0065] A silicon block polished by the method of the present inventionwas sliced into silicon wafers by a known method. With the thus obtainedsilicon wafers, a solar cell panel was manufactured and the crackingreduction ratio in the solar cell panel was obtained with respect tothat of a solar cell panel manufactured by a conventional method.Surface roughness Ry of 20 μm was determined as a reference for thecracking reduction ratio.

[0066] Sets of 10,000 silicon wafers having the surface roughness Ry of0.1, 1, 2, 4, 6, 8, 10, and 20 μm, respectively, were manufactured andsolar cell modules were manufactured through a solar cell modulemanufacture line. FIG. 4 shows the results. In FIG. 4, the surfaceroughness Ry (μm) is plotted in a vertical axis and the crackingreduction ratio (fold) of the solar cell panel is plotted in ahorizontal axis.

[0067] In the range of Ry=6-8 μm, reduction of cracked defective wafersof 1.5 or more was observed. That is, the surface roughness Ry of 8 μmor less is effective in reduction of cracked defective wafers.

Example 8

[0068] As shown in FIG. 4, a rectangular polysilicon ingot (250 mm inlength) was cut into silicon blocks 1 in the form of quadratic prism(125×125 mm) using a band saw 20. If the band saw has enough accuracy,it is not necessary to grind the surface of the silicon block. Edges 21of the silicon block 1 were cut off and rounded to complete the siliconblock.

[0069] Surfaces of the thus obtained silicon block that would serve ascircumferential surfaces of the silicon wafer were polished mechanicallyand physically by the method of the invention. Then, as shown in FIG. 5,the silicon block 1 was sliced with a wire saw (not shown) to obtainabout 470 silicon wafers 46.

[0070] The present invention provides a polishing technique forflattening the fine roughness on the side face of the silicon block orthe silicon stack in a short period and allows reduction of crackeddefective the silicon wafer and improvement in yield of the siliconwafer.

What is claimed is:
 1. A method of manufacturing a silicon wafer comprising the step of flattening fine roughness existing on a side face of a silicon block or a silicon stack used for manufacturing the silicon wafer.
 2. A method according to claim 1, wherein the step of flattening comprises spraying a mixture of abrasive grains and a medium on the side face of the silicon block or the silicon stack, shifting closer or contacting a polishing member to or with the side face to be polished, and moving the silicon block or the silicon stack relatively to the polishing member in the presence of the abrasive grains so that the side face of the silicon block or the silicon stack is mechanically and physically polished.
 3. A method according to claim 1, wherein the step of flattening comprises spraying a medium on the side face of the silicon block or the silicon stack, shifting closer or contacting a polishing member having abrasive grains on its surface and/or in the inside thereof to or with the side face to be polished, and moving the silicon block or the silicon stack relatively to the polishing member so that the side face of the silicon block or the silicon stack is mechanically and physically polished.
 4. A method according to claims 2 or 3, wherein the polishing is carried out while spraying the mixture of the abrasive grains and the medium or the medium solely.
 5. A method according to any one of claims 1 to 4, wherein the flattened side face of the silicon block or the silicon stack has surface roughness Ry of 8 μm or less.
 6. A method according to claim 5, wherein the flattened side face of the silicon block or the silicon stack has surface roughness Ry of 6 μm or less.
 7. A method according to any one of claims 1 to 6, wherein a section of the silicon block or the silicon stack is constructed of four main lines, the lines forming an angle of about 90° with adjacent lines, respectively. 